Firefox why window.opener is defined even for a new tab

I am porting quite old corp application from IE to Firefox. Here is my situation.

User goes away from computer for a while and his session becomes expired.

When user tries to access something in popup, he is being redirect开发者_C百科ed to login page (in popup window). In login page onload event popup window chain is being closed and root page refreshed (in root page tab user is redirected to login page).

The guy before me wrote this like this: if window.top.opener is undefined, we're not in popup. It worked fine in IE but I am getting non-stoppable refresh in Firefox.

Why in Firefox window.opener is still defined and points to same window, even if this is a new tab, not popup? How should I correctly determine that current window is popup?

Do not open new windows. Use some layers on page to show that message. Look at thickbox or something like that.